A light Emitting Diode (LED) may directly convert the electronic energy into optical energy. A LED chip is constituted of two portions, one of which is a P-type semiconductor in which a hole dominates and the other of which is an N-type semiconductor in which an electron dominates. When these two semiconductors are connected to each other, a “P-N junction” is formed between them. When a current is applied across the chip by a wire, the electrons are pushed towards the P region in which the electrons recombines with the holes to radiate energy in a form of photons. This is the principle of emission of LED.
As a novel light source, LED experiences an unprecedented development since its inapproachable advantages over conventional light sources, such as power saving, environmental, long life time, rapid start-up speed and capability of inhibiting sizes of the luminescent spectrum and the forbidden band width so that colors are rich. At present, the white LEDs are widely used in the field of illumination. In general, there are two major means for generating a white light, one of which utilizes three monochrome diodes which emit red, green and blue lights respectively to be mixed to generate a white light, the other of which utilizes a luminescent material to convert the monochrome light emitted by a blue or ultraviolet LED to a white light with a wide spectrum. The white light generated by the first means is mainly applied for a large screen display and that generated by the second means is mainly applied for illumination and a backlight source.
For the latter one, a Japanese company of Nichia possesses a groundbreaking invention (U.S. Pat. No. 5,998,925A), which utilizes a Blue GaN chip to excite a YAG yellow phosphor powder to obtain a white light. The inventive point of such a patent focus on a Yttrium Aluminum Garnet (YAG) phosphor which absorbs a blue light with a wavelength ranging from 450 nm to 470 nm and emits a yellow light with a wavelength ranging from 550 nm to 560 nm, and has an advantage of low cost and a high efficiency. For a normal blue or ultraviolet Led chip, its optoelectronic converting efficiency is generally lower than 30%. Even for the currently best LED chip, its optoelectronic converting efficiency is not higher than 50%. Thus, the LED emitting light accompanies with a lot of heat. At the position surrounding the lighten chip, the temperature will reach 150 to 200° C. Such a temperature may leads to a decrease of 20-30% for the efficiency of the phosphor, which in turn generates a shift of the color temperature for the light source with respect to the color coordinate, and affects the luminous efficiency and stability of the LED light source. The Yttrium Aluminum Garnet (YAG) phosphor degrades at a temperature above 120° C. Since the coated phosphor material is an opaque material, there is a scattering absorption when the light emitted by the blue or ultraviolet chip passes through the YAG phosphor, which leads to a lower emitting efficiency. Due to the uneven thickness of the coating, it seriously affects the light spot and the color temperature of the white light. For example, there are problems such as a yellow aperture, a blue light spot and inconsistency of the color temperature of the white light.
At present, the GaN based LED epitaxial wafer mostly utilizes a sapphire as the material for the substrate. Although the sapphire has a crystalline structure identical to that of GaN (hexago symmetrical wurtzite crystalline structure), it has a large lattice mismatch with the GaN material, i.e. up to 13%, which leads to a high dislocation density in the GaN epitaxial layer up to 108-109/cm2. The high dislocation density causes a decrease of luminous efficiency and leads to a mass of thermal dissipation, thereby causing light attenuation of the chip.